Graphite powder is a promising filler (i.e., conductive additive) for thermally and electrically conductive polymers and other composite materials.
Expanded or exfoliated graphite, also known as nanographite or nano-structured graphite, has recently attracted increased interest because of its excellent thermal and electrical conductivity properties. Expanded graphite outperforms non-expanded graphite and other conductive fillers (e.g., boron nitride, carbon fibers, carbon nanotubes) in terms of the thermal conductivity conveyed to polymers or other materials such as cement or gypsum-based materials. Adding expanded graphite to flooring materials to increase the thermal conductivity of the composite material is generally known in the art and has, for example, been described in DE-OS-100 49 230 A1.
However, disadvantages of adding expanded graphite—as opposed to conventional highly crystalline synthetic and natural graphite—to the polymer mass are its difficult workability and processability, its lower lubricating properties, its lower oxidation resistance, and its dustiness. In addition, processing expanded graphite in polymer compounders may result in flow problems that make it difficult to extrude the polymer including the expanded graphite. Problems in particular arise during the feeding of the expanded graphite into the extruder.
US 2009/0189125 to Grigorian et al. describes a process for preparing electrically conductive polymer composites comprising mixing non-predispersed carbon with an emulsion comprising a polymer in a liquid solvent to obtain a dispersion of the carbon in the polymer matrix, followed by removing the solvent from the dispersion (“solution compounding”). Grigorian et al. also describe as a comparative example a process on a laboratory scale wherein expanded graphite was mixed with dry polypropylene powders by mechanical mixing (compounding) followed by molding the mixture into composite sheets. Grigorian et al. does not describe any processability issues observed for expanded graphite, such as problems related to feeding of the expanded graphite into the extruder.
In addition, US 2007/031704 assigned to SGL Carbon describes conductive additives for gypsum materials comprised of compacted expanded graphite particles made from ground graphite foils. The expanded graphite is first compressed into large two dimensional structures (i.e. graphite foils) having a thickness of between 0.1 and 3 mm and a density between 0.8 and 1.8 g/cm3 and is then chopped, in a cutting mill, into smaller particles having a diameter between 1 and 5 mm and a bulk density of typically between 0.12 and 0.25 g/cm3. The resulting particles differ in their properties compared to the present invention, particularly in terms of the hardness of the particles, which is substantially higher in the particles described in US 2007/031704. In particular, the hardness of the expanded graphite particles described in US 2007/031704 has a negative effect on the thermal conductivity and on the mechanical properties of the composite product as compared to powdered expanded graphite.
EP 0 735 123 A describes processes for making graphite composite materials based on expanded graphite and used in chemical heat pumps or treatment devices for industrial gases. In the process described in EP 0 735 123 A, the expanded graphite is pre-densified into a macroscale matrix or laminate by compression or lamination. The composite compact is then further processed by impregnation and subsequent drying, followed by a final compression step to bring the graphite product into its final desired form. EP 0 735 123 A does not describe the use of the graphite as a conductive additive, e.g. for polymer products.
US 2008/0279710 A1 by Zhamu et al. describes a method of producing electrically conductive composite compositions particularly useful as fuel cell bipolar plates. The method comprises blending expandable (as opposed to expanded) graphite powder with non-expandable graphite powder and a binder, followed by expansion of the expandable graphite by heat treatment. Subsequently, the mixture is compressed into macroscale preformed composite compacts such as sheets and blocks which are then treated to activate the binder in the composition resulting in the desired composite plates that can be used in fuel cells (see, e.g., the flow chart in FIG. 2a). The patent application appears to describe the expansion of the expandable graphite portion of a mixture comprising expandable graphite, non-expandable graphite and a binder, and subsequent curing of the mixture by combined compression and binder treatments leading to composites of good mechanical integrity while exhibiting high transversal electrical conductivity. Zhamu et al. are not concerned with already expanded graphite powders, but rather prepare mixtures with binders to produce directly the desired composite compacts as the result of their process. While US 2008/0279710 A1 notes that expanded graphite is difficult to handle as concerns mixing with other powders such as non-expandable graphite, the solution presented to this problem is rather to mix the two graphite powders prior to exfoliation of the expandable graphite, thereby circumventing the problem of difficult handling due to the low density of expanded graphite.
Accordingly, it is an object of the invention to provide expanded graphite forms that preserve the excellent thermal and electrical conductivity of powdered expanded graphite while offering good processability comparable to standard, i.e. non-expanded synthetic or natural graphite. It is a further object to provide a process for preparing such advantageous expanded graphite forms and furthermore to provide composites comprising such advantageous expanded graphite forms. Finally, it is yet another object to provide applications and uses of conductive polymers comprising said advantageous expanded graphite forms.